Mid-Latitude Weather Hazards

While tropical storms dominate hazard headlines, the mid-latitudes (broadly 30–60 degrees N and S) experience a wide range of severe weather hazards. These include extratropical storms, tornadoes, heatwaves, cold spells, and extreme precipitation events. The UK, situated between 50–60 degrees N, is particularly affected by mid-latitude weather systems. Understanding these hazards requires knowledge of the polar jet stream, Rossby waves, depressions, anticyclones and their associated weather patterns.

---

Mid-Latitude Depressions (Extratropical Cyclones)

Key Definition: A mid-latitude depression is a low-pressure weather system that forms at the polar front, where warm subtropical air meets cold polar air. Depressions are the primary weather-producing systems in the mid-latitudes.

Formation (Cyclogenesis)

Mid-latitude depressions form through the process of cyclogenesis at the polar front, as described by the Norwegian Polar Front Model (Bjerknes and Solberg, 1922):

1. Cold polar air and warm tropical air meet at the polar front, creating a sharp temperature gradient (the baroclinic zone)
2. A wave develops along the front, typically initiated by upper-troposphere divergence associated with the jet stream
3. Warm air begins to rise over the cold air (the warm front) and cold air undercuts the warm air (the cold front)
4. The warm sector (between the two fronts) narrows as the cold front moves faster than the warm front
5. Eventually, the cold front catches up with the warm front, creating an occluded front and the depression begins to fill (weaken)

graph LR
    subgraph "Depression Structure - plan view"
        A["Cold air behind<br/>cold front"] --> B["Cold Front<br/>Heavy rain<br/>Cumulonimbus"]
        B --> C["Warm Sector<br/>Mild, cloudy<br/>Light rain"]
        C --> D["Warm Front<br/>Steady rain<br/>Stratus/nimbostratus"]
        D --> E["Cold air ahead<br/>of warm front"]
    end

Weather Associated with a Depression

Front/Sector	Cloud Types	Precipitation	Wind	Temperature
Approach of warm front	High cirrus, then altostratus, then nimbostratus	Steady rain, becoming heavier; may last 6–12 hours	Strengthening southerly/southeasterly	Cold, but rising slowly
Warm sector	Low stratus, stratocumulus	Light drizzle or dry	Moderate southwesterly	Mild and humid
Cold front passage	Towering cumulonimbus	Heavy rain, possibly with thunder; short duration (1–3 hours)	Veering to westerly/northwesterly; squally gusts	Sharp temperature drop
Behind cold front	Clearing; cumulus in unstable air	Showers; bright intervals	Fresh northwesterly	Cold and showery

---

Mid-Latitude Storms

Mid-latitude depressions can occasionally intensify into severe storms, especially when the jet stream is strong and focused:

UK Storm Events

Storm	Date	Impacts
Great Storm	15–16 October 1987	Winds gusted to 196 km/h (Gorleston); 18 deaths in England; 15 million trees uprooted; $2.3 billion in insured losses; famously not forecast by BBC forecaster Michael Fish
Burns' Day Storm	25 January 1990	Gusts up to 173 km/h; 47 deaths across UK; followed only 27 months after the Great Storm
Storm Ciara	8–9 February 2020	Part of a rapid succession of named storms; gusts to 150 km/h; widespread flooding; 3 deaths
Storm Arwen	26–27 November 2021	Gusts to 161 km/h; 3 deaths; one million homes lost power; 4,000 homes without power for over a week

"Sting Jet" Phenomenon

Some of the most damaging mid-latitude storms feature a meteorological phenomenon called a sting jet — identified by Keith Browning (2004) at the University of Reading. A sting jet is a narrow jet of air that descends from the cloud head of an intensifying depression, producing an intense and localised band of extreme winds (sometimes exceeding 160 km/h) that lasts 2–4 hours. It explains why some storms produce unexpectedly severe localised damage.

The Great Storm of 1987 is now believed to have featured a sting jet, which contributed to the extreme and localised wind damage across southern England.

---

Tornadoes

Key Definition: A tornado is a violently rotating column of air extending from a cumulonimbus cloud to the ground, typically visible as a funnel cloud. Tornadoes are the most intense small-scale atmospheric phenomenon.

Formation

Tornadoes form in environments with:

1. Strong wind shear — change in wind speed and/or direction with altitude creates a horizontal rotating tube of air
2. Intense updraft — a powerful thunderstorm updraft tilts the horizontal rotation into the vertical
3. Instability — warm, moist air at the surface beneath cooler air aloft creates strong convective potential
4. A trigger — a front, dryline, or outflow boundary lifts the warm air, initiating the thunderstorm

Supercell thunderstorms — the most organised and long-lived type of thunderstorm — are the primary producers of strong and violent tornadoes. They are characterised by a persistent rotating updraft called a mesocyclone.

The Enhanced Fujita Scale (EF Scale)

Rating	Wind Speed (km/h)	Damage	Frequency
EF0	105–137	Light: some roof damage, shallow-rooted trees blown over	~53% of US tornadoes
EF1	138–178	Moderate: roofs stripped, mobile homes overturned	~32%
EF2	179–218	Considerable: roofs torn off frame houses, large trees snapped	~10%
EF3	219–266	Severe: storeys destroyed, heavy cars thrown	~4%
EF4	267–322	Devastating: well-built structures levelled, cars thrown considerable distances	~1%
EF5	> 322	Incredible: strong frame houses swept away, steel-reinforced concrete badly damaged	< 0.1%

Distribution